Crude oil such as that produced from an undersea hydrocarbon reservoir, is usually stored in tanks in the vessel hull. Components of the crude oil evaporate and fill the upper spaces of the tanks. It is important to keep oxygen out of the tanks, to avoid a fire. When crude oil is removed from the tanks so the level of crude oil falls, hydrocarbon-inert gas (gas that does not react with hydrocarbons) is flowed into the tanks to prevent the entrance of air. When crude oil flows into the tanks so the level of crude oil rises, gas is removed from the tanks. The removed, or vented gas includes evaporated hydrocarbons in addition to the inert gas.
The vented gas, which is volatile because of the hydrocarbons in it, can be simply released into the atmosphere. However, if the vessel is moored so it does not continually move, then on calm days when there is little wind there is a danger that the vented gas will accumulate on the vessel deck and present a hazard. A system that was versatile in the maintenance of oxygen-free gas above crude oil in vessel tanks and in the safe disposal of vent gas containing hydrocarbon gas, would be of value.